The absolute best source of free tree seeds you can get is the
local bird population. Drive a post (even just a simple bamboo pole would work) into the ground where you think you want a tree. The birds will plant locally adapted fruiting trees and shrubs and give them a starter packet of fertilizer while they're at it. The more frequently you visit the property, the more easily you can weed out all but your favorite species.
For more traditional crops, I think most stone fruits (peach, plum, apricots) come easily from seed. If you're eating them anyways, it's cheap to plant a lot until you get some that can thrive in your climate. If you have access to fruit trees in your area, there are also a lot that can be grown from cuttings.